The present invention relates to sampling techniques for analysis of gaseous constituents in which a gas sample is collected in a sample bag.
A gas diluting and testing apparatus is used to analyze, among other things, vehicular exhaust. The apparatus uses a mixing tee to dilute the exhaust gasses so that the moisture content of the gasses is sufficiently reduced in order to minimize errors due to condensation. Existing mixing tees have a dilution inlet for receiving a dilution gas, a gaseous inlet for receiving the exhaust gasses, and a mixing portion where the gasses are brought together to form a diluted exhaust gas mixture.
In some systems, a sample of the diluted exhaust gas mixture is routed directly to external equipment, such as an analysis unit. Many times, a sample of the dilution gas is also sent directly to an analysis unit so that the subsequent analysis can more accurately determine what content is due to the exhaust gasses and what content is due to the dilution gas.
Because the hydrocarbon content of vehicle emissions is becoming smaller and smaller due to advances in technology, sometimes motivated by strict emission requirements, extra special care must be taken to assure that the diluted exhaust gas mixture is analyzed properly. Many factors that at one time presented negligible errors in the analysis of hydrocarbon (and other) constituent content in vehicular emissions are now significant because the constituent concentration is at such a low level, but must be measured with accuracy and precision. Of course, the problem of additional factors contributing to measurement error when the constituent to be measured has an exceedingly small concentration is not only present in the field of vehicle emission testing, the problem may also be present in other areas where gasses are analyzed to determine constituent content where the constituents of interest have very small concentrations.
As mentioned above, some approaches for measuring and analyzing gaseous constituent content in a test gas mixture use a mixing tee to dilute the gas mixture (for example, vehicle exhaust) by adding a dilution gas to the test gasses, producing a resultant mixture of test gasses and the dilution gas. The resultant mixture is then analyzed. In addition, the dilution gas may be analyzed so that content in the resultant mixture due to the test gasses can be distinguished from content in the resultant mixture due to the dilution gas. Some other measurement techniques use a different approach. In a different approach, the gasses, instead of being sent directly to an analysis unit, are collected in sample bags. Gasses are only kept in the sample bags for a short period of time, before the sample bags are connected to an analyzer. A gas collecting system that collects gasses in sample bags still faces the same problem of additional factors contributing to measurement error.
One existing system that uses sample bags to collect gas samples uses a sample bag made of polyvinylfloride (PVF) resin. The PVF sample bag has been used for many applications that have been commercially successful. However, the manufacturing process used to manufacture the PVF sample bag results in a small amount of hydrocarbons mixed in the PVF sample bag walls. When a gas sample (dilution gas or gas mixture such as exhaust gasses and dilution gas) is collected in the sample bag, hydrocarbons from the bag may slightly contaminate the gas sample. In many applications, the amount of hydrocarbon content contributed by the PVF sample bag to the gas sample is negligible when compared to the hydrocarbon content of the gas in the sample bag. As such, in many applications for exhaust gas analysis in which hydrocarbon content is to be measured, the PVF sample bags are suitable. However, as the hydrocarbon content within the gas sample becomes increasingly smaller, due in part to strict emission requirements for vehicles, the contribution of hydrocarbons to the gas sample from the PVF sample bag becomes a significant source of measurement error.
For the foregoing reasons, there is a need for an improved gas sample bag that reduces hydrocarbon contribution to the gas sample from the sample bag, without introducing other significant errors so that the overall integrity of the sample is increased.
It is, therefore, an object of the present invention to provide a sample bag for collecting a sample of gaseous constituents in which the sample chamber wall allows limited but significant diffusion of a gaseous constituent therethrough, and an auxiliary chamber is defined between the sample chamber and outside environment to limit the diffusion rate out of the sample chamber to an acceptable level.
In carrying out the above object and other objects and features of the present invention, a sample bag for collecting a sample of gaseous constituents is provided. The sample bag comprises an inner bag and an outer bag. The inner bag defines a sample chamber and has a wall that bounds the sample chamber. The wall is made of a material that allows limited diffusion through the wall of at least one of the gaseous constituents. The outer bag defines an auxiliary chamber at least partially bound by the wall such that limited diffusion between the sample chamber and the auxiliary chamber is allowed through the wall.
It is appreciated that the limited diffusion through the sample chamber wall is not a desirable result, but is a consequence for certain materials that may be selected for the wall. The present invention provides various configurations for a sample bag that substantially reduce any undesirable diffusion through the wall that may occur due to the selected material. Further, it is appreciated that it is not desirable to have a large number of diffusing constituents. But again, the present invention provides various configurations for a sample bag that substantially reduce any undesirable diffusion.
In one embodiment, the wall material is polytetrafluoroethylene (PTFE). Of course, the list of films that may be used in the alternative to PTFE is almost endless. In a preferred embodiment, the outer bag substantially encloses the inner bag such that the auxiliary chamber substantially surrounds the sample chamber. More preferably, the inner bag wall generally bounds the auxiliary chamber with respect to the sample chamber, and the outer bag is generally surrounded by an outside environment. Preferably, the outer bag includes an outside wall that bounds the auxiliary chamber with respect to the outside environment. The outside wall of the outer bag may be made of a material that allows limited diffusion through the outside wall of at least one of the gaseous constituents, for example, the outside wall material may also be polytetrafluoroethylene (PTFE).
Preferably, a first inlet is provided for filling the inner bag, and a second inlet is provided for filling the outer bag. Advantageously, the inner and outer bags may be filled with the same sample material, with the inner bag holding the actual sample to be analyzed, while the same sample material in the outer bag limits diffusion out of the inner bag to an acceptable level even though constituents may diffuse to the outside environment from the outer bag.
In some embodiments, the sample bag further comprises an additional bag defining an additional chamber. The outer bag has an outside wall that bounds the auxiliary chamber and the additional chamber is at least partially bound by the outer bag outside wall such that diffusion between the auxiliary chamber and the additional chamber takes place through the wall.
Further, in carrying out the present invention, a sample bag for collecting a sample of gaseous constituents comprises a plurality of barriers defining a plurality of nested chambers. The plurality of barriers defines the plurality of nested chambers and associated boundaries, including a sample chamber. A sample chamber boundary separates the sample chamber from at least one different chamber and is made of a material that allows limited diffusion through the sample chamber boundary of at least one of the gaseous constituents. The at least one different chamber has an outside wall that bounds the at least one different chamber with respect to an outside environment.
In a preferred implementation, the outside wall is made of a material that allows limited diffusion through the outside wall of at least one of the gaseous constituents such that diffusion from the sample chamber to the outside environment passes through the at least one different chamber. For example, the at least one of the barriers may be made of polytetrafluoroethylene (PTFE). The plurality of nested chambers may define an inner bag containing the sample chamber and an outer bag containing the at least one different chamber, or may define any other suitable group of nested chambers such that diffusion out of the actual sample chamber is limited to an acceptable level to preserve the integrity of the sample.
Still further, in carrying out the present invention, a method for collecting a sample of gaseous constituents from a source is provided. The method comprises collecting the sample in an inner bag, and collecting a control gas in an outer bag. The inner bag defines a sample chamber and has a wall that bounds the sample chamber. The wall is made of material that allows limited diffusion through the wall of at least one of the gaseous constituents in the sample. The outer bag defines an auxiliary chamber at least partially bound by the wall such that diffusion between the sample chamber and the auxiliary chamber takes place through the wall. The outer bag has an outside wall that bounds the auxiliary chamber with respect to an outside environment. The outer bag outside wall is made of material that allows limited diffusion through the outer bag outside wall of at least one of the gaseous constituents.
It is appreciated that embodiments of the present invention use a material to provide a barrier around the sample chamber wherein the material allows limited diffusion through the barrier of a gaseous constituent causing the relative concentrations of the constituents to vary. To preserve the integrity of the sample within the sample chamber, any number of additional chambers are defined about the sample chamber. These additional chambers are filled with a control gas, which may be the same gaseous mixture being sampled in the sample chamber. The gaseous constituent diffuses out of the one or more auxiliary chambers to the outside environment, as the same gaseous constituent diffuses from the sample chamber to the auxiliary chamber or chambers. Because the gaseous constituent content in the sample chamber and in the auxiliary chamber are relatively similar concentrations, diffusion out of the sample chamber is slow, preserving the integrity of the sample.
Of course, the inner bag wall and the outer bag outside wall may be made of essentially the same material having the same essential properties, or may be made of different materials. Further, the control gas for the outer bag need not be from the same source as the sample for the inner bag, and may be from a different source so long as the gaseous constituent content for the constituent that diffusion is to be controlled is sufficiently close to the content in the inner bag.
Even further, in carrying out the present invention, a sample bag for collecting a sample of gaseous mixture for emissions testing is provided. The mixture is a mixture of exhaust gasses and a dilution gas. The sample bag comprises an inner bag defining a sample chamber having an outer wall that bounds the sample chamber. The outer wall is made of material that allows limited diffusion through the outer wall of exhaust gasses. The sample bag further comprises an outer bag substantially enclosing the inner bag and defining an auxiliary chamber at least partially bound by the inner bag outside wall. Diffusion between the sample chamber and the auxiliary chamber is allowed through the inner bag outside wall. The outer bag has an outer bag outside wall made of material that allows limited diffusion through the outer bag outside wall of exhaust gasses to an external environment.
The above object, and other objects, features, and advantages of the present invention are readily apparent from the following detailed description of the best mode for carrying out the invention when taken in connection with the accompanying drawings.